Low-temperature storage tank provided with pump well structure for reducing dead zone liquid level and operation method of low-temperature storage tank

Abstract

The invention discloses a low-temperature storage tank provided with a pump well structure for reducing the liquid level of a dead zone and an operation method of the low-temperature storage tank. The storage tank comprises a storage tank body. At least one first pump well structure is arranged in the storage tank body, each first pump well structure comprises a first pump well pipe, a first immersed pump capable of being completely immersed below the liquid level is arranged at the bottom end of the interior of each first pump well pipe, and a negative pressure protection cover is arranged at an inlet of each first pump well pipe. The negative pressure protection cover is used for establishing a negative pressure cavity to reduce the liquid level of a dead zone and avoid vortex bubbles when the first immersed pump discharges liquid, and the top of the negative pressure protection cover can be communicated with the gas phase space in an openable and closable manner; the storage tank body is communicated with an air pressure adjusting device capable of adjusting the operation pressure of the gas phase space. The unique negative pressure protection cover is arranged, a negative pressure cavity is formed in the outer side of the first immersed pump, and vortexes and bubbles are prevented from being sucked in; the storage tank body is communicated with the air pressure adjusting device, the operation range of operation pressure in the gas phase space is expanded, the dead zone liquid level height of the storage tank is reduced, the effective volume of the storage tank is increased, and tank cleaning residual liquid is extracted to the maximum extent.

Description

Technical Field

[0001] The present invention relates to the technical field of cryogenic storage tanks, and in particular to a cryogenic storage tank provided with a pump well structure for reducing a dead zone liquid level and an operating method thereof. Background Art

[0002] Atmospheric pressure storage tanks for low-temperature media such as LNG are the most critical core equipment for receiving stations, peak-shaving stations and other stations, and occupy an important position in the energy storage system. According to the difference in structure, vertical flat-bottom storage tanks can be divided into single-capacity tanks, double-capacity tanks, full-capacity tanks, membrane tanks and other tank types. This type of cryogenic storage tank can hold liquids such as cryogenic LNG, cryogenic ethylene, cryogenic ethane, cryogenic liquid ammonia, cryogenic propane, cryogenic propylene, and cryogenic butane at near normal pressure.

[0003] At present, the main type of LNG cryogenic storage tank is a large prestressed concrete fully contained storage tank with a capacity of 100,000 to 270,000 cubic meters. Due to the low construction cost per unit volume of storage tanks, obvious scale effect, and the increase in tank capacity can also more efficiently improve land utilization and reduce the BOG evaporation rate of storage tanks, LNG storage tanks are constantly developing in the direction of large-scale.

[0004] According to the specification, large cryogenic full-volume storage tanks should not have holes in their walls or bottoms, and pipelines connected to the tanks should generally enter and exit from the top of the tanks. Since the tanks have a large diameter (the largest exceeds 110m) and a high height (the highest is about 50m), and the cryogenic medium in the tank is close to its saturated vapor pressure under the storage environment, a vacuum pump cannot be used to extract materials, and only a cryogenic submersible pump immersed below the liquid level can be used. For information on related pumps, see "CN103615394B Vertical Cryogenic Tank Submersible Pump".

[0005] When the cryogenic submersible pump is started, the minimum liquid level in the tank must not be lower than the minimum operable liquid level required by the cryogenic submersible pump. At present, the minimum operable liquid level of the cryogenic submersible pump is usually about 1.2m to 2.4m after adding a certain safety margin, that is, there is a 1.2m to 2.4m high liquid at the bottom of the tank that cannot be drained, which becomes the liquid level "dead zone" of the tank, accounting for about 3% to 6% of the tank volume. As the diameter of the tank continues to increase, the dead zone volume of the tank also increases, resulting in increased costs and waste of resources.

[0006] Since the materials in the dead zone cannot be discharged from the tank by the cryogenic submersible pump, when the tank needs to be cleaned, these bottom materials can only be discharged by vaporization, which takes a long time, consumes a lot of energy, and is not conducive to tank shutdown and maintenance. The reasons for not being able to completely clean the tank are that the height of the pump well bottom valve is too high, cavitation, vortex is easily sucked in to cause surge, and the submersible pump needs to be fully immersed below the liquid level to achieve good heat dissipation.

[0007] The solution adopted in the prior art is to dig a groove at the bottom of the storage tank to form a liquid pool. The submersible pump is placed in the liquid pool to achieve the function of lowering the liquid level in the dead zone of the storage tank. The problems it has are as follows:

[0008] (1): After the tank bottom is grooved, the integrity of the concrete cap supporting the tank needs to be destroyed, which is very bad for the cap structure. Especially under earthquake conditions, there is a risk of cap cracking. It is necessary to eliminate the risk by increasing the amount of cap reinforcement, which increases the cost of the tank cap.

[0009] (2): After the tank bottom is grooved, in order to ensure that the heat outside the tank is not transferred into the tank through the liquid pool, it is necessary to lay insulation materials on the bottom and surrounding of the liquid pool after the groove. This will cause large temperature difference stress, high construction difficulty and poor insulation effect.

[0010] (3): After the tank bottom is grooved, two layers of metal materials, the secondary bottom plate and the inner tank bottom plate, need to be laid on the upper surface of the insulation material. The grooved area is a special-shaped structure, and the sealing performance is poor when laying the steel plate, which makes construction difficult. When the metal material processed into a special-shaped structure encounters working conditions such as thermal expansion and contraction, there is also the risk of tearing and leakage.

[0011] (4): The above structure has high costs and great safety risks, and the final benefits may not offset the investment.

[0012] The solution adopted in the second prior art is to use a Venturi mixer, which requires a series of complex process systems such as a Venturi mixer, a material circulation tank, a cryogenic pump, a control valve, a pressurizing unit, etc., and has a complex structure, high cost, and poor reliability.

[0013] Therefore, the inventor proposes a cryogenic storage tank provided with a pump well structure for reducing the dead zone liquid level and an operating method thereof to overcome the defects of the prior art. Summary of the invention

[0014] The object of the present invention is to provide a low-temperature storage tank provided with a pump well structure for lowering the dead zone liquid level and an operating method thereof. The present invention is provided with a unique negative pressure protection cover, and the outer side of the first submersible pump constitutes a negative pressure chamber, and a stable liquid level is established around the submersible pump through the negative pressure chamber to avoid the inhalation of vortices and bubbles; the storage tank body is connected to an air pressure regulating device to expand the operating range of the operating pressure in the gas phase space, thereby achieving "avoiding cavitation" and "the first submersible pump is completely immersed below the liquid level", reducing the dead zone liquid level height of the low-temperature storage tank, increasing the effective volume of the storage tank, and extracting the residual liquid from the tank to the greatest extent.

[0015] The objective of the present invention is achieved in this way. A low-temperature storage tank provided with a pump well structure for lowering the dead zone liquid level comprises a storage tank body; at least one first pump well structure is arranged in the storage tank body, the first pump well structure comprises a first pump well pipe which is sealed and penetrated from the top of the storage tank body, a first submersible pump which can be completely immersed below the liquid level is arranged at the inner bottom end of the first pump well pipe, a negative pressure protection cover which is sealed at the top and open at the bottom is arranged at the bottom inlet of the first pump well pipe, the negative pressure protection cover is used to establish a negative pressure chamber to lower the dead zone liquid level and avoid vortex bubbles when the first submersible pump discharges liquid, the storage tank body is located between the liquid level and the tank top to form a gas phase space, the top of the negative pressure protection cover can be opened and closed to connect to the gas phase space; an air pressure regulating device which can adjust the operating pressure in the gas phase space is connected to the storage tank body, the air pressure regulating device can increase the operating pressure in the gas phase space so that the static pressure of the liquid at the first submersible pump is greater than the saturated vapor pressure of the liquid.

[0016] In a preferred embodiment of the present invention, a drainage skirt structure is provided at the bottom end of the negative pressure protective cover, and the drainage skirt structure is used to absorb residual liquid, collect vortex bubbles, and stabilize the inlet flow field.

[0017] In a preferred embodiment of the present invention, the top end of the negative pressure protection cover is connected to the first end of the external connecting pipe, and the second end of the external connecting pipe passes through the tank body and then returns to enter the gas phase space of the tank body; a first valve is provided at a position of the external connecting pipe located outside the tank body, and the first valve is used to control the connection state between the negative pressure protection cover and the gas phase space.

[0018] In a preferred embodiment of the present invention, a plurality of second pump-well structures are further provided in the storage tank body, each of the second pump-well structures comprises a second pump-well pipe, and a second submersible pump is provided at the bottom end of the second pump-well pipe.

[0019] In a preferred embodiment of the present invention, when performing a tank cleaning operation, the operating pressure of the gas phase space in the tank body is greater than or equal to the saturated vapor pressure of the low-temperature medium in the operating environment, and is less than or equal to the upper limit of the pressure that the tank body can withstand.

[0020] In a preferred embodiment of the present invention, the interval between the inner wall of the negative pressure protection cover and the outer wall of the first pump well pipe ranges from 1mm to 10000mm; the sum of the heights of the negative pressure protection cover and the drainage skirt structure ranges from 500mm to 12000mm.

[0021] In a preferred embodiment of the present invention, a plurality of ribs and rib supports are arranged at intervals inside the negative pressure protection cover.

[0022] In a preferred embodiment of the present invention, the first pump well pipe and the negative pressure protection cover are connected to the top and inner wall of the storage tank body through a pump well support structure.

[0023] In a preferred embodiment of the present invention, the drainage skirt structure is connected to the bottom plate of the storage tank body through a bottom supporting structure.

[0024] The object of the present invention can also be achieved in that a method for operating a cryogenic storage tank provided with a pump well structure for reducing the dead zone liquid level comprises the following steps:

[0025] Step a, feeding operation: the top of the negative pressure protective cover is connected to the gas phase space in the storage tank body, the air pressure in the negative pressure protective cover is consistent with the air pressure in the gas phase space, and the internal space of the negative pressure protective cover is filled with liquid; the pressure of the pipeline outside the tank connected to the first pump well pipe is controlled, the liquid level in the first pump well pipe is equal to the operating liquid level in the storage tank body, and the first submersible pump is completely immersed below the liquid level;

[0026] Step b, discharging operation: the top of the negative pressure protection cover is disconnected from the gas phase space in the storage tank body, the first submersible pump is started to discharge liquid outward, and the operating liquid level in the storage tank body continues to decrease; when the operating liquid level in the storage tank body decreases to a level close to the cavitation level of the pump, the air pressure regulating device is started to inject air into the storage tank body to increase the operating pressure of the gas phase space to a first pressure value, the pressure at the bottom inlet of the first pump well pipe is higher than the cavitation pressure of the pump, and the first submersible pump continues to discharge liquid outward;

[0027] Step c: tank cleaning operation:

[0028] The air pressure regulating device maintains the operating pressure of the gas phase space at a first pressure value, and the first submersible pump continuously discharges liquid outward. When the operating liquid level in the storage tank body drops to close to the minimum tank cleaning level, the tank cleaning is completed.

[0029] As described above, the cryogenic storage tank provided with a pump well structure for reducing the dead zone liquid level and the operating method thereof of the present invention have the following beneficial effects:

[0030] In the present invention, the air pressure regulating device is connected to the storage tank body, which breaks the limitation of the conventional operating pressure fluctuation range of 4 to 7 kPa in the low-temperature storage tank, and expands the operating range of the operating pressure in the gas phase space through the air pressure regulating device, so as to achieve "avoiding cavitation" and "the first submersible pump is completely immersed below the liquid level", reduce the dead zone liquid level height of the low-temperature storage tank, increase the effective volume of the storage tank, and extract the residual liquid in the tank to the maximum extent; a unique negative pressure protection cover is provided, and the outer side of the first submersible pump constitutes a negative pressure chamber, and a stable liquid level is established around the submersible pump through the negative pressure chamber to avoid the inhalation of vortices and bubbles; the present invention does not need to modify the base of the storage tank, does not need to add complex process equipment, does not change the performance parameters of the existing submersible pump, does not change the tank bottom and the main structure of the inner tank wall of the storage tank, and makes full use of the existing process equipment. It is suitable for all types of low-temperature storage tanks that use submersible pumps to discharge liquid, and has a simple structure, low investment and good effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The following drawings are only intended to illustrate and explain the present invention, and are not intended to limit the scope of the present invention.

[0032] in:

[0033] Figure 1 : It is an internal structure diagram of embodiment 1 of the low-temperature storage tank provided with a pump well structure for reducing the dead zone liquid level of the present invention.

[0034] Figure 2 : It is a structural diagram of the first pump well structure and the second pump well structure of the present invention.

[0035] Figure 3 : It is a structural diagram of the first pump well structure of the present invention.

[0036] Figure 4 : It is a schematic diagram of the feeding operation of Example 1 of the present invention.

[0037] Figure 5 : is a schematic diagram of the discharging operation of Example 1 of the present invention.

[0038] In the figure:

[0039] 1. First pump well structure; 11. First pump well pipe; 12. First submersible pump;

[0040] 2. Second pump well structure; 21. Second pump well pipe; 22. Second submersible pump;

[0041] 3. Storage tank body; 30. Gas phase space;

[0042] 4. Negative pressure protection cover; 41. First valve; 42. External connecting pipe;

[0043] 5. Drainage skirt structure. DETAILED DESCRIPTION

[0044] In order to have a clearer understanding of the technical features, purposes and effects of the present invention, specific embodiments of the present invention are now described with reference to the accompanying drawings.

[0045] The specific embodiments of the present invention described herein are only used to explain the purpose of the present invention and cannot be understood as limiting the present invention in any way. Under the guidance of the present invention, technicians can conceive of any possible variations based on the present invention, which should be considered to belong to the scope of the present invention. It should be noted that when an element is referred to as "arranged on" another element, it can be directly on the other element or there can also be a central element. When an element is considered to be "connected" to another element, it can be directly connected to the other element or there may be a central element at the same time. The terms "installed", "connected" and "connected" should be understood in a broad sense, for example, it can be a mechanical connection or an electrical connection, or it can be the internal communication of two elements, it can be directly connected, or it can be indirectly connected through an intermediate medium. For ordinary technicians in this field, the specific meanings of the above terms can be understood according to the specific circumstances. The terms "vertical", "horizontal", "up", "down", "left", "right" and similar expressions used herein are only for illustrative purposes and do not represent the only implementation method.

[0046] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those generally understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0047] like Figures 1 to 5 As shown, the present invention provides a low-temperature storage tank provided with a pump well structure for reducing the dead zone liquid level, including a storage tank body 3, and the storage tank body 3 can adopt an existing low-temperature storage tank, that is, the main structure of the storage tank, such as a support, an insulation structure, an inner tank bottom, an inner tank wall (existing technology), etc., is not changed;

[0048] At least one first pump well structure 1 is arranged in the tank body 3, such as Figure 3 , Figure 4 , Figure 5As shown, the first pump well structure 1 includes a first pump well pipe 11 which is sealed and penetrated from the top of the storage tank body 3, and a first submersible pump 12 which can be completely immersed below the liquid level is arranged at the inner bottom end of the first pump well pipe 11. A negative pressure protection cover 4 which is sealed at the top and open at the bottom is arranged at the bottom entrance of the first pump well pipe 11. The negative pressure protection cover 4 is used to establish a negative pressure chamber when the first submersible pump 12 discharges liquid to reduce the dead zone liquid level and avoid vortex bubbles. A gas phase space 30 is formed between the liquid level and the tank top in the storage tank body 3, and the top of the negative pressure protection cover 4 can be opened and closed to communicate with the gas phase space 30 of the storage tank body 3;

[0049] The storage tank body 3 is connected to an air pressure regulating device which can adjust the operating pressure in the gas phase space 30. The air pressure regulating device can increase the operating pressure in the gas phase space 30 so that the static pressure of the liquid at the first submersible pump 12 is greater than the saturated vapor pressure of the liquid, thereby avoiding cavitation and ensuring that the first submersible pump 12 is completely immersed below the liquid level.

[0050] The first submersible pump 12 can use an existing submersible pump, that is, the performance parameters of the existing submersible pump do not change, and there is no need to force the submersible pump itself to lower its minimum suction level. While meeting the existing submersible pump's minimum suction level of 1.2 to 2.4 m, all liquids above a level of 0.15 m can still be pumped out of the tank body 3.

[0051] The gas pressure regulating device of the present invention can increase the operating pressure in the gas phase space 30 so that the static pressure of the liquid at the first submersible pump 12 is greater than the saturated vapor pressure of the liquid, thereby avoiding cavitation.

[0052] The present invention reduces the inhalation of excessive vortices, bubbles, etc. during the tank cleaning process, and avoids the submersible pump from vibrating violently due to the inhalation of gas. The present invention can also be used to empty the residual liquid in the storage tank to the maximum extent, and compared with the traditional vaporization tank cleaning solution, the tank cleaning cycle is shortened from one to two months to about one week.

[0053] The first submersible pump 12 of the present invention is completely immersed below the liquid level to avoid problems with heat dissipation of the submersible pump.

[0054] The present invention makes full use of existing process equipment, and does not need to configure additional large-scale process equipment such as a supercharger. The gas pressure regulating device of the present invention can use the BOG supercharger (existing technology), gasifier (existing technology) and related pipeline devices that are standardly equipped at the receiving station, and can be used in conjunction with the storage tank body 3 of the present invention, without the need for additional process equipment for the storage tank.

[0055] The invention is applicable to all types of low-temperature storage tanks that use submersible pumps to discharge liquid from the top, such as single-capacity tanks, double-capacity tanks, full-capacity tanks, and membrane tanks. The improved structure can also be used for storage tanks that discharge liquid from the bottom, such as crude oil storage tanks, finished oil storage tanks, etc.

[0056] In the cryogenic storage tank provided with a pump well structure for reducing the dead zone liquid level of the present invention, the tank body 3 is connected to an air pressure regulating device, which will break the normal operating pressure fluctuation limit of 4 to 7 kPa in the cryogenic storage tank, and expand the operating range of the operating pressure in the gas phase space 30 through the air pressure regulating device, so as to achieve "avoiding cavitation" and "the first submersible pump 12 is completely immersed below the liquid level", reduce the dead zone liquid level height of the cryogenic storage tank, increase the effective volume of the storage tank, and extract the residual liquid in the tank to the maximum extent; a unique negative pressure protective cover 4 is provided, and the outer side of the first submersible pump 12 constitutes a negative pressure chamber, and a stable liquid level is established around the submersible pump through the negative pressure chamber to avoid the inhalation of vortices and bubbles; the present invention does not need to modify the base of the storage tank, does not need to add complex process equipment, does not change the performance parameters of the existing submersible pump, does not change the tank bottom, the main structure of the inner tank wall, and makes full use of existing process equipment. It is suitable for all types of cryogenic storage tanks that use submersible pumps to discharge liquid, and has a simple structure, low investment, and good effect.

[0057] Further, if Figure 1 , Figure 2 , Figure 3 As shown, a drainage skirt structure 5 is provided at the bottom end of the negative pressure protection cover 4, and the drainage skirt structure 5 is used to absorb residual liquid, collect vortex bubbles, and stabilize the inlet flow field. The lower end of the drainage skirt structure 5 needs to be open to guide the LNG liquid to the inlet of the first pump well structure 1 in an orderly manner. The flow cross-sectional area of ​​the gap between the drainage skirt structure 5 and the bottom plate of the storage tank body 3 needs to be larger than the cross-sectional area of ​​the pump well to ensure stable drainage.

[0058] The three functions of the drainage skirt structure 5 are as follows: first, it forms a space filled with liquid together with the negative pressure protective cover 4 to ensure the stability of the inlet flow field of the submersible pump; second, it has a drainage function, and the minimum liquid level can be adjusted by adjusting the height of the lower edge of the drainage skirt structure 5; third, before the first submersible pump 12 inhales vortices and bubbles, the gas can be collected in time and temporarily guided to the negative pressure chamber (when the vortex and bubbles are inhaled, the vortex and bubbles are guided along the inner wall of the drainage skirt structure 5 to the interval above the liquid level of the negative pressure chamber L3), avoiding the first submersible pump 12 from surging due to the inhalation of the vortex. Furthermore, a set of vortex breakers (existing technology) can be installed on the lower edge of the drainage skirt structure 5 to further reduce the risk of inhaling the vortex.

[0059] Further, if Figure 1 , Figure 2 , Figure 3 As shown, the top end of the negative pressure protection cover 4 is connected to the first end of the external connecting pipe 42, and the second end of the external connecting pipe 42 passes through the tank body 3 and returns to enter the gas phase space 30 of the tank body; the external connecting pipe 42 is located at the outside of the tank body 3 to set a first valve 41, and the first valve 41 is used to control the connection state between the negative pressure protection cover 4 and the gas phase space 30.

[0060] The first valve 41 is located outside the tank body 3, which is convenient for workers to operate. The first end of the external communication pipe 42 is connected to the top of the negative pressure protection cover 4, and the second end of the external communication pipe 42 passes through the tank body 3 and then returns to the gas phase space 30 of the tank body. In this way, the worker can establish and break the pressure difference between the negative pressure protection cover and the gas phase space 30 in the tank body 3 by closing and opening the valve outside the tank body 3.

[0061] When the tank body 3 is filled with liquid, the first valve 41 is opened to ensure that the LNG liquid fills the inner space of the negative pressure protection cover 4. When the tank body 3 is discharged, the first valve 41 needs to be closed to ensure that a negative pressure cavity is established in the annular space between the negative pressure protection cover 4 and the first pump well pipe 11.

[0062] If the liquid level in the negative pressure protection cover 4 drops seriously due to the inhalation of excessive bubbles and vortices, affecting the stability of the flow field at the inlet of the submersible pump, it can be operated from outside the tank to use the first valve 41 and the external connecting pipe 42 to extract the gas affecting the vacuum degree of the negative pressure chamber, so as to maintain the pressure difference and compensate the vacuum degree to the maximum extent.

[0063] Further, if Figure 1 , Figure 2 As shown, a plurality of second pump-well structures 2 are also arranged in the storage tank body 3, each of which includes a second pump-well pipe 21, and a second submersible pump 22 is arranged at the bottom end of the second pump-well pipe 21. The second pump-well structure 2 adopts a conventional pump-well structure, and the second submersible pump 22 and the first submersible pump 12 can adopt submersible pumps with the same parameters.

[0064] Under the traditional structure, the submersible pump needs to suck LNG liquid from the bottom valve at the bottom of the pump well. At this time, the dead zone liquid level is generally not less than 1.2 to 2.4 meters. When the impeller of the submersible pump rotates, LNG liquid will be continuously pumped into the pump well pipe, the liquid level in the pump well pipe rises, and finally flows out of the outlet at the top of the pump well pipe to the outside of the tank, completing the liquid discharge operation.

[0065] Under normal pressure, LNG is a methane liquid at about -162℃, with a saturated vapor pressure of about 0.1MPa(A). When the pressure is lower than 0.1MPa(A), the LNG liquid will boil and vaporize, and cannot remain in liquid state. Therefore, when extracting LNG liquid from the storage tank, a vacuum pump cannot be used, only a submersible pump can be used.

[0066] The reason why the submersible pump can extract LNG liquid is that the impeller of the submersible pump provides initial kinetic energy for the LNG liquid in the pump well pipe. Under the action of kinetic energy, the LNG can be delivered to the top of the pump well and discharged from the tank, so that the LNG liquid in the pump well does not need to be lower than its saturated vapor pressure. But even so, the submersible pump still has the risk of cavitation: because the impeller of the submersible pump rotates very fast, the LNG flow rate in contact with the blades is also very large, causing the LNG pressure in the local area near the impeller to be lower than its saturated vapor pressure, forming severe cavitation and destroying the blade structure. In order to solve the risk of cavitation, the minimum suction level of the submersible pump needs to maintain a certain height, such as 1.2~2.4m mentioned above. This level is the minimum level to prevent cavitation in the submersible pump. When the operating liquid level of the storage tank is lower than this level, the submersible pump under the traditional structure is difficult to operate normally and cannot be started at any time, thus causing a large dead zone liquid level.

[0067] In addition, the motor of the submersible pump will generate heat during operation and needs to be continuously cooled. Therefore, the submersible pump must be continuously immersed in LNG to maintain heat dissipation performance. This also requires the minimum liquid level to be maintained at a certain height, generally higher than the upper edge of the pump.

[0068] Furthermore, during the tank cleaning operation, the operating pressure of the gas phase space 30 in the tank body is greater than or equal to the saturated vapor pressure of the low-temperature medium in the operating environment, and less than or equal to the upper limit of the pressure that the tank body 3 can withstand.

[0069] When the dead zone liquid level needs to be drained, the operating pressure P2 of the gas phase space 30 above the tank liquid level in the tank body 3 should always be maintained in the range of 10 to 29 kPa (depending on the different tank types). Considering the overall safety performance of the tank, the preferred range is 10 to 15 kPa.

[0070] Furthermore, a certain space needs to be reserved between the inner wall of the negative pressure protection cover 4 and the outer wall of the first pump well pipe 11. The spacing between the inner wall of the negative pressure protection cover 4 and the outer wall of the first pump well pipe 44 is in the range of 1mm to 10000mm, preferably between 50mm and 1000mm.

[0071] The upper end of the negative pressure protection cover 4 is sealed and connected to the outer wall of the first pump well pipe 11 by welding or the like to ensure strength and air tightness.

[0072] Under the condition that the flow area is not less than the flow area required by the pump well, the lower edge of the drainage skirt structure 5 should be made as close as possible to the minimum tank cleaning liquid level L1 required for tank cleaning. The preferred L1 liquid level range is 150-1500 mm.

[0073] The sum of the heights of the negative pressure protection cover 4 and the drainage skirt structure 5 is selected according to different design requirements and working conditions. The sum of the heights of the negative pressure protection cover and the drainage skirt structure ranges from 500mm to 12000mm, preferably between 1000 and 6000mm.

[0074] Furthermore, a plurality of ribs and rib supports are arranged at intervals inside the negative pressure protection cover 4 to prevent the negative pressure protection cover 4 from becoming unstable due to negative pressure.

[0075] Furthermore, the first pump well pipe 11 and the negative pressure protection cover 4 are connected to the top and inner wall of the tank body through the pump well support structure. The pump well support structure is similar to the traditional pump well structure, some of which limit 6 degrees of freedom and some limit 3 degrees of freedom.

[0076] Furthermore, the drainage skirt structure 5 is connected to the bottom plate of the tank body 3 through a bottom support structure to reduce the risk of vibration when sucking liquid.

[0077] Further, the air pressure regulating device regulates the operating pressure in the gas phase space 30 in the storage tank body 3 in the following manners:

[0078] (1) Under normal operating conditions, the liquid in the tank body 3 (LNG tank) evaporates naturally, which automatically increases the operating pressure P2 of the gas phase space 30. When P2 is too high, a certain amount of gas can be extracted from the tank through the gas pressure regulating device (BOG compressor) to reduce the pressure P2; by controlling the amount of gas extracted, the pressure difference value can be quantitatively controlled.

[0079] (2) When the pressure of the gas phase space 30 needs to be increased rapidly, P2 can be increased rapidly by adding gas to the tank through a gas pressure regulating device outside the tank (BOG compressor, gasifier such as open rack gasifier ORV, submerged gasifier SCV, intermediate medium gasifier IFV, etc.).

[0080] Embodiment 1

[0081] The cryogenic storage tank provided with the dead zone liquid level reducing pump well structure of the present invention is used as a prestressed concrete fully contained LNG storage tank, and three second pump well structures 2 and one first pump well structure 1 are arranged in the storage tank body 3. Multiple first pump well structures 1 can also be arranged, so that when one of them fails, the remaining first pump well structures 1 can be started as a backup.

[0082] In the low-temperature storage tank that only uses the conventional pump well structure, i.e., the second pump well structure 2, the gas phase space 30 enclosed by the storage tank body 3 (concrete outer tank) is filled with gasified NG gas (natural gas, mainly methane) above the liquid level. The maximum operating pressure P2 (gas gauge pressure) in the gas phase space 30 is about 29 kPa (depending on the type of tank), and is generally maintained between 4 and 7 kPa (depending on the type of tank). The method for maintaining the gas pressure is: after the liquid in the storage tank body 3 evaporates naturally, P2 will gradually increase. Once P2 exceeds 7kPa, the gas pressure regulating device (BOG compressor) is started to extract BOG gas (Boil-Off Gas, referred to as BOG, refers to the gas evaporated by absorbing external heat when the low-temperature liquid is liquefied by pressurization below its critical temperature due to the difficulty in absolute insulation from the environment); once it is detected that P2 is lower than 4kPa, BOG gas can be transported into the tank through the gas pressure regulating device (BOG compressor or vaporizer and other equipment), so as to maintain the operating pressure in the tank stable within the range of 4 to 7kPa.

[0083] The low-temperature storage tank of the present invention is provided with a pump well structure for lowering the dead zone liquid level. In addition to the conventional pump well structure, namely the second pump well structure 2, a first pump well structure 1 is also provided. A negative pressure protection cover 4 and a drainage skirt structure 5 are provided on the outside of the first pump well pipe 11 of the first pump well structure 1. In the present embodiment, the interval between the inner wall of the negative pressure protection cover 4 and the outer wall of the first pump well pipe 44 is 200 mm, and the sum of the heights of the negative pressure protection cover and the drainage skirt structure is 3000 mm.

[0084] A negative pressure chamber is formed between the inner wall of the negative pressure protection cover 4 and the outer wall of the first pump well pipe 11. The pressure in the negative pressure chamber is P1, that is, the saturated vapor pressure 0.1 MPa (A); P2 is the operating pressure of the gas phase space 30, and P2 is 0 to 29 kPa higher than P1.

[0085] L4 is the liquid level in the first pump well pipe 11, L3 is the liquid level in the negative pressure chamber (h3 is the distance from L3 to the bottom plate of the tank body 3), L2 is the operating liquid level in the tank body 3 (h0 is the distance from L2 to the bottom plate of the tank body 3, h1 is the distance from L3 to L2 when L2 is lower than L3, i.e. h3=h0+h1), L2 is the lower surface of the bottom valve (existing technology) in the traditional pump well structure, L1 is the lowest tank cleaning liquid level that can be achieved by the present invention (lowest liquid level, h2 is the distance from L1 to the bottom plate of the tank body 3). L5 is the bottom valve liquid level of the submersible pump (the first submersible pump 12 and the second submersible pump 22).

[0086] In this embodiment, the height of the first submersible pump 12 is about 2 m; to avoid cavitation, the minimum liquid level to prevent pump cavitation must not be less than 1.5 m.

[0087] To achieve "avoiding cavitation" and "submerging the submersible pump below the LNG liquid level", the following conditions must be met:

[0088] (1) The liquid level L3 should be higher than the upper surface of the first submersible pump 12, that is, h3>2m+h2, so that the submersible pump can be completely immersed below the LNG liquid level.

[0089] (2) L3 liquid level h3 should be higher than 1.5m+h2.

[0090] At the height of the liquid level L1 at the inlet of the pump well pipe, the Bernoulli equation is established on the outside and inside of the negative pressure chamber respectively to obtain:

[0091] P2+ρg(h0-h2)+0.5*ρv 2 2 =P1+ρg(h3-h2)+0.5*ρv 1 2

[0092] The average flow velocity v1 in the negative pressure chamber is set to 0.454 m / s, and the flow velocity v2 outside the negative pressure chamber can be approximately simplified to 0. The density of LNG is 480 kg / m 3 .thus:

[0093] △P=P2-P1=4708.8(h3-h0)+49.5

[0094] △P can be adjusted within the range of 0 to 29 kPa, so the range of h1 = (h3-h0) can be obtained; at the same time, L2 is set as low as the lowest tank cleaning level L1, that is, h0 = h2 = 150 mm. It is calculated that when △P ≥ 10 kPa, h3 = 2.263 m. At this time, not only can "submerge the submersible pump below the LNG level" and "avoid cavitation" be achieved, but the L2 level can also be reduced to 0.15 m to achieve the goal of reducing the dead zone level and extreme tank cleaning.

[0095] Under the above operating conditions, the LNG storage tank of the present invention can start and stop the cryogenic pump at any time when L2 is lower than 1.5m, and is not affected by cavitation.

[0096] In this embodiment, the operating pressure P2 (gas gauge pressure) of the gas phase space 30 in the storage tank body 3 is assumed to be P2=10kPa, the flow velocity V1 in the first pump well pipe 11 is 0.454m / s; the cavitation liquid level of the first submersible pump 12 is set to 1.5m. In this embodiment, the dead zone liquid level is reduced from 1.5m to 0.15m. The operation method of the present invention will be described below through a complete feeding and discharging operation process, which specifically includes the following steps:

[0097] Step a, feeding operation: the top of the negative pressure protection cover 4 is connected to the gas phase space 30 in the storage tank body 3, so that the air pressure in the negative pressure protection cover 4 is consistent with the air pressure in the gas phase space 30 in the storage tank body 3, and the internal space of the negative pressure protection cover 4 is smoothly filled with liquid; the pressure of the pipeline outside the tank connected to the first pump well pipe 11 is controlled, the liquid level in the first pump well pipe 11 is substantially equal to the operating liquid level in the storage tank body 3, and the first submersible pump 12 is completely immersed below the liquid level;

[0098] Specifically include:

[0099] When the storage tank body 3 is fed, the first valve 41 outside the storage tank body 3 is opened, and the top of the negative pressure protection cover 4 is connected to the gas phase space 30 inside the storage tank body 3 to ensure that the LNG liquid fills the internal space of the negative pressure protection cover 4. At this time, the negative pressure cavity of the negative pressure protection cover 4 is full of liquid, and the L3 liquid level reaches the maximum value (the maximum height of the negative pressure protection cover 4 in this embodiment is 3m, so L3 = 3m), and the L2 operating liquid level will be much higher than L3. In this embodiment, the maximum value of L2 is set to 30m; Figure 4 As shown, during the feeding operation, the top of the first pump well pipe 11 is connected to the gas phase space 30, then the liquid level L4 in the first pump well pipe 11 is approximately equal to L2, and the first submersible pump 12 is completely immersed below the liquid level, and can be started at any time to perform the liquid discharge operation.

[0100] Step b, discharging operation: the first valve 41 outside the storage tank body 3 is closed, the top of the negative pressure protection cover 4 is disconnected from the gas phase space 30 in the storage tank body 3, the first submersible pump 12 is started to discharge liquid outward, and the operating liquid level in the storage tank body 3 continues to decrease; when the operating liquid level in the storage tank body 3 decreases to a level close to the cavitation level of the pump, the air pressure regulating device is started to inject air into the storage tank body to increase the operating pressure of the gas phase space 30 to a first pressure value, the pressure at the bottom inlet of the first pump well pipe 11 is higher than the cavitation pressure of the pump, and the first submersible pump 12 continues to discharge liquid outward;

[0101] Specifically include:

[0102] Before the liquid discharge operation is performed, the first valve 41 on the outside of the storage tank body 3 needs to be closed in advance to ensure that the negative pressure chamber is filled with LNG liquid as much as possible.

[0103] like Figure 5 As shown, after starting the first submersible pump 12, the L2 liquid level will continue to decrease from a height of 30m. When the L2 liquid level decreases to a level close to the pump cavitation level (the pump cavitation level in this embodiment is 1.5m, and L2 is close to 4m at this time), the gas pressure regulating device (BOG compressor or vaporizer) is started to inject boil-off gas into the tank body 3, so that the operating pressure P2 of the gas phase space 30 is maintained at about 10kPa.

[0104] When the L2 liquid level continues to drop to about 0.887m, the L3 liquid level also starts to drop from a height of 3m, and a liquid level difference of 2.113m will always be maintained between the L3 liquid level and the L2 liquid level (2.113m+0.887m=3m).

[0105] At this time, if the first submersible pump 12 is kept running without stopping, the liquid level L4 in the first pump well pipe 11 will always be full of the pump well pipe; since the P2 pressure is always maintained at 10kPa, the pressure at the bottom inlet of the first pump well pipe 11 is much higher than the cavitation pressure of the pump, which meets the requirements of "avoiding cavitation" and "submerging the submersible pump below the LNG liquid level", and the first submersible pump 12 can continue to work;

[0106] At this time, if the first submersible pump 12 is temporarily stopped, the liquid level L4 in the first pump well pipe 11 will be consistent with the liquid level L3 in the negative pressure chamber. Since the minimum value of L3 is h3=2.263m, it can also meet the requirements of "avoiding cavitation" and "immersing the submersible pump below the LNG liquid level". The pump can be started at any time to continue to extract the dead zone liquid level.

[0107] In contrast, if the conventional pump well structure stops the pump at this time, the L4 liquid level is likely to be lower than the 1.5m cavitation level. Once the pump is stopped, it cannot be restarted, and the residual liquid cannot be pumped out. This is also one of the advantages of the present invention over the conventional pump well.

[0108] Step c: tank cleaning operation:

[0109] The air pressure regulating device maintains the operating pressure of the gas phase space at a first pressure value, and the first submersible pump 12 continuously discharges liquid outward. When the operating liquid level in the storage tank body 3 drops to close to the minimum tank cleaning level, the tank cleaning is completed.

[0110] Specifically include:

[0111] When the L2 liquid level drops to close to 0.15m (i.e., the L1 liquid level), h3=2.663m, which still meets the requirements of "avoiding cavitation" and "submerging the submersible pump below the LNG liquid level", and the tank cleaning is completed.

[0112] It can be seen from the above operation that the present invention can reduce the dead zone liquid level from 1.5m of the traditional pump well structure to 0.15m (0.15m is the minimum value allowed by the specification).

[0113] For a tank with a diameter of 84.2m, the effective volume will be increased by about 7513m 3 , the volume will be increased by 3.75%; for a 270,000 cubic meter storage tank with a diameter of about 110m, the effective volume will be increased by about 12823m 3 , the volume increased by 4.75%.

[0114] As described above, the cryogenic storage tank provided with a pump well structure for reducing the dead zone liquid level and the operating method thereof of the present invention have the following beneficial effects:

[0115] In the present invention, the air pressure regulating device is connected to the storage tank body to break the normal operating pressure fluctuation limit of 4 to 7 kPa in the low-temperature storage tank, and the operating range of the operating pressure in the gas phase space is expanded through the air pressure regulating device to achieve "avoiding cavitation" and "the first submersible pump is completely immersed below the liquid level", reduce the dead zone liquid level height of the low-temperature storage tank, increase the effective volume of the storage tank, and extract the residual liquid in the tank to the maximum extent; a unique negative pressure protection cover is provided, and the outer side of the first submersible pump constitutes a negative pressure chamber, and a stable liquid level is established around the submersible pump through the negative pressure chamber to avoid the inhalation of vortices and bubbles; the present invention does not need to modify the base of the storage tank, does not need to add complex process equipment, does not change the performance parameters of the existing submersible pump, does not change the tank bottom, the main structure of the inner tank wall, and makes full use of existing process equipment. It is suitable for all types of low-temperature storage tanks that use submersible pumps to discharge liquid, and has a simple structure, low investment, and good effect.

[0116] The above description is only an illustrative embodiment of the present invention and is not intended to limit the scope of the present invention. Any equivalent changes and modifications made by any person skilled in the art without departing from the concept and principle of the present invention shall fall within the scope of protection of the present invention.

Claims

1. A cryogenic storage tank provided with a pump well structure for reducing the dead zone liquid level, characterized in that: It comprises a storage tank body; at least one first pump-well structure is arranged in the storage tank body, the first pump-well structure comprises a first pump-well pipe which is sealably penetrated from the top of the storage tank body, a first submersible pump which can be completely immersed below the liquid level is arranged at the inner bottom end of the first pump-well pipe, a negative pressure protection cover which is sealed at the top and open at the bottom is arranged at the bottom entrance of the first pump-well pipe, the negative pressure protection cover is used to establish a negative pressure chamber when the first submersible pump discharges liquid to reduce the dead zone liquid level and avoid vortex bubbles, the storage tank body is located between the liquid level and the tank top to form a gas phase space, the top of the negative pressure protection cover can be opened and closed to connect to the gas phase space; the storage tank body is connected to an air pressure regulating device which can regulate the operating pressure in the gas phase space, the air pressure regulating device can increase the operating pressure in the gas phase space so that the static pressure of the liquid at the first submersible pump is greater than the saturated vapor pressure of the liquid.

2. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 1, characterized in that: A drainage skirt structure is arranged at the bottom end of the negative pressure protective cover, and the drainage skirt structure is used for absorbing residual liquid, collecting vortex bubbles and stabilizing the inlet flow field.

3. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 2, characterized in that: The top end of the negative pressure protective cover is connected to the first end of the external connecting pipe, and the second end of the external connecting pipe passes through the tank body and then returns to the gas phase space of the tank body; a first valve is arranged at a position of the external connecting pipe outside the tank body, and the first valve is used to control the connection state between the negative pressure protective cover and the gas phase space.

4. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 2, characterized in that: A plurality of second pump-well structures are also arranged in the storage tank body, each of the second pump-well structures comprises a second pump-well pipe, and a second submersible pump is arranged at the bottom end of the second pump-well pipe.

5. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 2, characterized in that: When performing the tank cleaning operation, the operating pressure of the gas phase space in the tank body is greater than or equal to the saturated vapor pressure of the low-temperature medium in the operating environment, and is less than or equal to the upper limit of the pressure that the tank body can withstand.

6. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 2, characterized in that: The interval between the inner wall of the negative pressure protection cover and the outer wall of the first pump well pipe ranges from 1mm to 10000mm; the sum of the heights of the negative pressure protection cover and the drainage skirt structure ranges from 500mm to 12000mm.

7. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 2, characterized in that: A plurality of rib plates and rib plate supports are arranged at intervals inside the negative pressure protection cover.

8. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 2, characterized in that: The first pump well pipe and the negative pressure protection cover are connected to the top and inner wall of the storage tank body through a pump well support structure.

9. The cryogenic storage tank with a pump well structure for reducing dead zone liquid level according to claim 2, characterized in that: The drainage skirt structure is connected to the bottom plate of the storage tank body through a bottom supporting structure.

10. An operating method for a cryogenic storage tank provided with a pump well structure for reducing dead zone liquid level according to any one of claims 1 to 9, characterized in that: The following steps are involved: Step a, feeding operation: the top of the negative pressure protective cover is connected to the gas phase space in the storage tank body, the air pressure in the negative pressure protective cover is consistent with the air pressure in the gas phase space, and the internal space of the negative pressure protective cover is filled with liquid; the pressure of the pipeline outside the tank connected to the first pump well pipe is controlled, the liquid level in the first pump well pipe is equal to the operating liquid level in the storage tank body, and the first submersible pump is completely immersed below the liquid level; Step b, discharging operation: the top of the negative pressure protection cover is disconnected from the gas phase space in the storage tank body, the first submersible pump is started to discharge liquid outward, and the operating liquid level in the storage tank body continues to decrease; when the operating liquid level in the storage tank body decreases to a level close to the cavitation level of the pump, the air pressure regulating device is started to inject air into the storage tank body to increase the operating pressure of the gas phase space to a first pressure value, the pressure at the bottom inlet of the first pump well pipe is higher than the cavitation pressure of the pump, and the first submersible pump continues to discharge liquid outward; Step c: tank cleaning operation: The air pressure regulating device maintains the operating pressure of the gas phase space at a first pressure value, and the first submersible pump continuously discharges liquid outward. When the operating liquid level in the storage tank body drops to close to the minimum tank cleaning level, the tank cleaning is completed.

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